In conventional electrolytic treatments typically including the continuous plating or acid cleaning of strips of steel plate, semi-expendable or insoluble electrodes such as ferrosilicon, lead, lead alloys, or the like have been used as the counterelectrode as disclosed in, for example, JP-B-53-18167 (the term "JP-B" as used herein means an "examined Japanese patent publication"). For example, in an electrolytic zinc-plating line, a steel plate is used as the cathode and a lead alloy is used as the counter anode, and the steel plate is advanced at a line speed as high as 1 to 2 m/sec, while the end of the steel plate is usually oscillating perpendicularly to the line direction. Furthermore, the distance between the steel plate and its counterelectrode is extremely short, i.e., about 10 mm on average, for the purposes of stable operation, cost saving, etc. Because of such electrolytic plating lines, there have often been cases where the steel plate being treated comes into contact with the counterelectrode to cause shortcircuiting. In the case of lead alloy electrodes, the shortcircuiting problem has not been so urgent because the lead alloy has a nature that even if it fuses due to the heat generated at the time of shortcircuiting, the fused part of the lead alloy immediately absorbs the heat as heat of fusion and returns to its solid state. However, the lead alloy electrode is defective in that the amount of lead dissolved from the electrode into the electrolyte during electrolysis is as relatively large as 1 to 10 mg/AH, and the dissolved lead comes into the resulting platings on the products. For this reason, use of insoluble metal electrodes has come to be studied, which comprise substrates made of valve metals such as titanium, having formed over the substrate surfaces coatings containing platinum group metals or oxides thereof as disclosed in, for example, JP-A-56-47597 (the term "JP-A" as used herein means an "unexamined published Japanese patent application"). This kind of electrode is characterized in that the erosion of the coating is very slow, i.e., about 1/10 to 1/100 of the lead electrodes, and they are substantially insoluble and dimensionally stable. Therefore, the insoluble metal electrodes are coming to be used extensively.
However, use of such a metal substrate electrode has a problem that if shortcircuiting as described above occurs, not only the coating but also the substrate is damaged. In order to prevent shortcircuiting, the material being treated such as steel plate has conventionally been prevented from coming into direct contact with the electrode surface by, for example, placing a net of fluoroplastic or other plastic or a plate of resin or ceramic on the surface of the electrode. However, if an edge of the steel plate etc. which is traveling at a high speed hits such a net, the net is cut too easily to be practically used. Although plates may retain their physical strength, use of plates is defective in that the electrode surface is masked to substantially limit the effective area of the electrode because the plate has a large area so as to protect the whole electrode surface, and this leads to a problem that the life of the electrode is shortened and unevenness of treatment results.
The above problems apply to electrolytic acid cleaning and similar treatments. Those problems have been severe particularly in high-speed treatments.